This invention relates generally to the field of electric motor driving systems, and more particularly to a method and apparatus for fast rotor position detection for disk drive motors at standstill.
Electric motors include a rotor and a stator having a plurality of wound field coils. Brushless DC motors are electronically commutated, wherein solid-state switching replaces the brushes and segmented commutators of traditional permanent magnet DC motors. Brushless motors may be of the variable reluctance, permanent magnet, or hybrid type. Variable reluctance brushless motors are characterized by having an iron core rotor follow or chase sequentially shifting magnetic fields of the stator coils to facilitate rotational motion of the rotor. Permanent magnet brushless motors are characterized by having the sequentially energized field coils attract or repel a permanent magnet rotor into rotational motion.
Electric motors are used to rotate loads in a variety of applications. One such application is in mass storage devices, such as hard disk drives. A hard disk drive generally includes a stack of rotating disks or platters and a spindle motor, which may be a salient pole brushless DC motor, for rotating the disks. The drive also includes one or more electromagnetic read/write heads which fly above the surface of the disks, an actuator motor (also known as a voice coil motor or VCM) which controls the positioning of the read/write heads, power circuitry to provide electrical power to the spindle and voice coil motors, circuitry for processing the data read from and/or written to the drive, and control circuitry to control the operation of the spindle and voice coil motors. The platters are typically rotated at a generally constant angular speed while the read/write heads read from or write to circular tracks on the platters. The mass storage device spindle motors are commonly multiple phase motors including a permanent magnet rotor and three electrical windings. The three electrical windings are related to the three phases of the motor. Three phase currents flow through the motor windings, typically at a 120 electrical degree phase relationship with respect to one another. The phase currents create a rotating electric field which causes angular rotation of the permanent magnet rotor.
The electromagnetic read/write heads read data from a disk platter by sensing flux changes on the magnetic surface of the platter as it passes beneath the read/write head. In order to synchronize the data being read from the disk with the operation of the data processing circuitry, it is necessary to carefully control the rotational speed of the disks. This is accomplished by controlling the current delivered to the spindle motor phase windings. The phase currents may be generated by the control circuitry in a variety of fashions. One method is to provide pulse-width-modulated (PWM) signals to the motor windings, wherein the timing of the individual PWM signals provided to each motor phase is determined by a control circuit. The duty cycle of the pulse width modulation signal therefore determines the average current delivered to the spindle motor. Another mode of current control is known as linear current control. The spindle motor control circuitry adjusts the level of current delivered by the power circuitry according to a desired motor performance parameter, such as speed and/or position.
Power is delivered to the motor phases through selectively energizing and de-energizing the individual phase windings. This process is known as commutation, and is accomplished via the control circuit. In order to rotate the disk drive motor in a given direction from startup and to maintain a desired rotational speed and torque at steady state, a commutation sequence or scheme is employed according to the present rotor position. This ensures that the proper phase windings are energized at appropriate times and polarities in order to provide the mutual attraction and/or repulsion between the phase windings and the rotor magnetic poles which results in the desired angular rotor motion.
To ensure proper rotational movement, it is essential to determine the position of the rotor with respect to the de-energized stator windings (or with respect to the energized windings). By knowing this position (sometimes referred to as commutation position), the stator windings can be energized in the appropriate sequence to create a revolving magnetic field in the motor to exert the desired rotational torque on the rotor. Rotor position has previously been detected by employing one or more transducers within the motor to sense the position of the rotor relative to the active stator windings.
However, the use of such transducers to determine commutation position has several drawbacks. First, these sensors increase production costs due to the need for sophisticated positional adjustment and increased wiring. Moreover, the space required for commutation position sensors is also a significant disadvantage in that valuable space is consumed within the motor housing. With an ever-increasing premium on space and cost efficiency, several attempts have been made to create sensorless commutation position feedback systems to replace the need for commutation position sensors within such motors.
The commutation control circuit is provided with rotor position information feedback (as well as rotor speed information), which is used to generate appropriate commutation signals for the motor windings. This position information may be obtained from some form of position sensor, or from measurements of back electromotive force (emf). Rotational position sensors include hall effect devices, magnetic sensors, optically encoded disks, resolvers, and other devices providing an indication of the relative positions of the rotor and stator to the control circuit via separate sensor signals. These devices, however, add to the cost and complexity of a motor, as well as occupying valuable physical space.
In mass storage devices such as high density disk drives, it is desirable to prevent a stationary spindle motor from starting in the wrong rotational direction. Accordingly, the rotor position is typically determined prior to moving the rotor, so that an appropriate commutation sequence may be employed at startup. In conventional disk drives, the stationary rotor position determination is typically accomplished by providing six sequential current pulses to the motor phases. These current pulses may be provided via the application of long duration voltage pulses which saturate the flux or field associated with the energized winding. For example, two such long duration voltage pulses of opposite polarity are commonly provided to each phase pair in a three phase motor.
The rise times of the resulting currents have been heretofore measured and compared in order to determine the rotor position within a given accuracy. For example, many such rotor position methods determine the rotor location by assuming that a motor phase winding with the shortest measured rise time associated therewith is the phase closest to a rotor magnetic pole. The current pulse method is based on saturation of the field or flux associated with a motor winding. The current pulse method therefore requires that the applied energy have a long enough duration to provide the saturation. When saturation occurs, the inductance of an energized winding decreases, resulting in a decreased rise time (e.g., related to L/R). The winding closest to a rotor pole will tend to saturate faster than will the other windings. The fastest rise time may be determined, for example, by measuring the time it takes for each current to rise to a certain threshold value. Alternatively, the resulting current with the largest or highest peak may be used to identify the winding closest to a rotor pole.
In a typical three phase motor, conventional stationary rotor position detection methods are able to determine the rotor position within 60 electrical degrees for a three phase motor. However, improved position detection accuracy is desirable. In addition, the prior current pulse method is time consuming. In advanced mass storage disk drive spindle motor applications, there is a need for faster motor startup. The current pulses of the prior method must be long enough to affect the motor flux saturation, in order to indicate rotor position. The six current pulse methods of conventional systems therefore do not provide for fast stationary rotor position detection.
The present invention provides an improved apparatus and methodology for determining the position of a stationary rotor in a polyphase electric motor which overcomes or minimizes the shortcomings of the prior art. In particular, the invention provides for a faster stationary rotor position determination than was possible in conventional systems. The method includes measuring a mutual inductance of the motor, and/or a voltage signal related to mutual inductance; and determining the stationary rotor position according to the measured mutual inductance. According to one aspect of the present invention, the mutual inductance measurement may be accomplished, for example, by applying a voltage pulse across a pair of phases and measuring a floating phase voltage. The inventive method includes very short voltage pulses (e.g., less than about 0.01 ms), which are significantly shorter than are the current pulses of the prior art.
For a three phase motor, the method may include measuring first, second, and third mutual inductances associated with the first, second, and third phases, respectively, and determining the rotor position according to the first, second, and third mutual inductances. The mutual inductances may be measured by applying a first voltage pulse across the second and third phases while measuring a first floating phase voltage across the first phase, applying a second voltage pulse across the third and first phases while measuring a second floating phase voltage across the second phase, and applying a third voltage pulse across the first and second phases while measuring a third floating phase voltage across the third phase.
The polarity of the measured floating phase voltages may also be measured, whereby determining the rotor position according to the measured mutual inductance comprises identifying a motor phase closest to a proximate rotor pole according to the floating phase voltage polarity measurements. The inventors of the present invention have found that the equivalent mutual inductance of a pair of energized phase windings with respect to a floating phase is a periodic function of rotor position at a given current range, having alternating positive and negative polarities. Accordingly, the application of a short voltage pulse creates a measurable voltage (e.g., related to the product of the mutual inductance and the di/dt created by the voltage pulse) having a polarity associated therewith. In this way, the floating phase voltage measurements, and/or a polarity associated therewith, may be correlated to one of six positional regions or rotor locations with respect to the stator phases. In addition, the six positional regions may each comprise two 30 degree electrical angular partitions on opposite sides of the motor.
According to another aspect of the invention, the method may further include injecting first and second current pulses or long duration voltage pulses of opposite polarity into a pair of motor phases, and measuring first and second rise times or peak vales associated with resulting first and second currents. From these measurements, the polarity of the proximate rotor pole may be determined according to the first and second rise times or peak values associated with the resulting first and second currents.
In accordance with yet another aspect of the invention, there is provided another method of detecting a stationary rotor position in a polyphase electric motor having first, second, and third phases. The method includes applying a first voltage pulse across the second and third phases while measuring a first floating phase voltage across the first phase, applying a second voltage pulse across the third and first phases while measuring a second floating phase voltage across the second phase, applying a third voltage pulse across the first and second phases while measuring a third floating phase voltage across the third phase, and determining the stationary rotor position according to the measured first, second, and third floating phase voltages.
The applied voltage pulses may have a duration of less than about 0.01 ms, and the polarities of the first, second, and third floating phase voltages may be measured. Using the measured polarities, one of the three phases may be identified as being closest to a proximate rotor pole. In this manner, the first, second, and third floating phase voltage polarity measurements may be correlated with one of six positional regions, which may represent two 30 degree electrical angular partitions on opposite sides of the motor. In this way, a fast rotor position detection methodology is provided, which achieves accurate results in less time than was previously possible using six current pulses.
According to still another aspect of the invention, an apparatus is provided for detecting a stationary rotor position in a polyphase electric motor having first, second, and third phases. The apparatus comprises a first circuit adapted to apply a first voltage pulse across the second and third phases and to measure a first floating phase voltage across the first phase. The first circuit is further adapted to apply a second voltage pulse across the third and first phases and to measure a second floating phase voltage across the second phase, as well as to apply a third voltage pulse across the first and second phases and to measure a third floating phase voltage across the third phase. The apparatus also includes a second circuit adapted to determine the stationary rotor position according to the measured floating phase voltages.
In this regard, the first circuit may be adapted further to measure the polarity of the floating phase voltages, whereby the second circuit may identify one of the first, second, and third phases as a motor phase closest to a proximate rotor pole according to floating phase voltage polarity measurements. This may be done by correlating the first, second, and third floating phase voltage polarity measurements with one of six positional regions, wherein the one of six positional regions represents two 30 degree electrical angular partitions on opposite sides of the motor.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.